The present invention relates to a load-dependent preventive fuse.
Fuses for electric circuits are known per se. They are used to protect the electric circuits as well as the devices and machines connected thereto in case of undesired, generally unforeseen, loads on said circuits, which could cause damage to THE circuits as well as to the devices and machines connected thereto. Such loads may be caused by voltage peaks in the mains, short-circuits caused by improper use of the devices, ambient temperatures which are too high or too high a degree of humidity etc. Accordingly, fuses, thermistors or humistors are used, either separately or in combination with each other. Such fuses are hereinafter also referred to as load fuses.
To limit overvoltages in electric circuits use can be made of a varistor as disclosed in, for example, EP-B1-0137044, in which the electrical properties of a semiconducting strontium titanate ceramic are used. Electrical properties of different ceramic crystals are known from, for example, the paper by R. Waser, entitled "dc Electrical Degradation of Perovskite-Type Titanate: II, Single Crystals", published in J. Am. Ceram. Soc. 73 (6) 1654-62 (1990).
In quite a number of cases, however, it should never come to the point where a fuse becomes operative. The electrical device along with the electric circuit should rather be selectively switched off after a certain operating time or duration of the load, i.e. it is preventively switched off. This frequently occurs in situations where machines must be serviced for safety reasons after specific load intervals . Such machines include, for example, car or aircraft engines. The necessity of preventively switching off a device can also be prompted by the likelihood that in the near future the device will break down and that exchanging it while it is still functioning is simpler or cheaper than exchanging it when it is defective. In other devices, such as light sources, a specific property, for example the brightness, decreases with length of service in such a manner that the device should be exchanged, although other properties such as the electrical resistance are still satisfactory. Such fuses are hereinafter also referred to as preventive fuses.
Important components of such preventive fuses are elapsed time meters, mileage indicators or fluid-flow meters. In general, a meter reading is used to activate an optical or acoustic warning signal, so that the device can be switched off manually or by means of a control unit. Preventive fuses whose activation depends only on the elapsed time will hereinafter be referred to as time-dependent preventive fuses.
In many cases, however, preventive fuses whose activation is triggered by a combination or specific constellation of measured physical parameters are advantageous. In these cases, logically interconnected preventive fuses are used. Frequently, not only the operating time but also the accumulated load of a machine during its operating time should be used as a criterion for maintenance intervals. This applies, for example, to car engines. To determine the maintenance intervals, in general, monitoring only the mileage reading is insufficient. The temporally accumulated engine load must also be registered. It is conceivable that as a result of traffic jams the engine is operated over a prolonged period of time while the reading of the odometer remains unchanged.
The load to which engines are subjected is often directly proportional to the operating temperature of the engines. That is, the higher the temperature of the engine, the higher the load to which it is subjected. For this reason, preventive fuses which simultaneously monitor the operating time, the temporally accumulated operating temperature and hence the temporally accumulated operating load as the activation criterion are advantageous. This kind of preventive fuses will hereinafter also be referred to as load-controlled preventive fuses. According to the present state of the art, this problem is solved by preventive fuses which are provided with separate meters for temperature and time. However, each individual meter has a failure risk attached to it. Consequently, the failure risk of the assembled preventive fuse is higher than that of each individual element. In addition, the cost of mounting such an assembly is higher than that of mounting an individual element.